Why were early direct-current power systems inefficient for transmitting electricity over long distances?

Early direct-current (DC) power systems were inefficient for long-distance transmission primarily because they could not easily change voltages.1

In electrical transmission, power loss is determined by the formula $P_{loss}=I^{2}R$, where $I$ is the current and $R$ is the resistance of the lines. Because early DC grids could not step up voltage, they were forced to operate at low voltages to maintain usable power, which necessitated a high current. This high current resulted in significant energy loss due to the resistance of the transmission wires.12

In contrast, alternating-current (AC) systems eventually supplanted DC because they could utilize transformers to increase voltage for transmission. By transmitting power at very high voltages, AC systems could maintain a correspondingly low current, thereby drastically reducing energy loss over long distances.12